This invention pertains to pulse-width extending optical systems and a projection-exposure apparatus using such optical systems. More specifically, the invention pertains to optical systems that extend the duration of light pulses emitted by a pulsed laser, and to projection-exposure apparatus using such optical systems. The invention also pertains to methods for manufacturing semiconductor devices using such projection-exposure apparatus.
FIG. 16 is a block diagram of a conventional microlithography projection-exposure apparatus comprising a pulsed laser. This projection-exposure apparatus comprises a laser 1 that emits light pulses. A beam-shaping optical system 2 shapes the cross section of the beam. The beam then enters a fly-eye lens 3. The fly-eye lens 3 divides the incident laser beam into multiple secondary light sources, one such secondary light source being formed at the rear focal point of the fly-eye lens 3. An aperture 4 limits the beam, and a condenser lens 5 uniformly illuminates a mask 6 with the beam. Typically the mask 6 contains a high-resolution pattern with extremely small features, e.g., patterns for semiconductor integrated circuits. A projection optical system 7 projects the pattern of the mask 6 on a wafer 8. The projected pattern may be either demagnified (reduced) or magnified (enlarged).
The resolution of the pattern of the mask 6 as projected on the wafer 8 depends on the wavelength of the light from the laser 1. The laser 1 emits light having as short a wavelength as possible in order to form high-resolution patterns on the wafer 8.
Many lasers emitting short wavelengths of light emit pulses of light. The peak optical powers and intensities of pulsed lasers are very much larger than their average optical powers. (Optical power is defined as optical energy per unit time; optical intensity is optical power per unit area.) For example, for an ArF excimer laser which emits light at a wavelength of 193 nm and which has a beam cross section of 20 mm by 5 mm, typical peak pulse intensities during pulses are on the order of 10 MW/cm2.
Short-wavelength radiation tends to cause changes in optical materials. These changes include increased absorption by the materials and radiation-induced changes in refractive index. These changes are frequently irreversible. In addition, such changes are more readily produced by high power and high-intensity radiation in comparison with radiation of similar average power but lesser peak values. For this reason, systems using short-wavelength lasers often suffer from radiation-induced changes to their optical elements.
Conventional projection-exposure apparatus using short-wavelength lasers also exhibit astigmatism caused by variations in the refractive indices of the lens material of the projection optical system. Such astigmatism significantly degrades the resolution of the projection optical systems.
For example, the ArF excimer laser (emission wavelength of 193 nm) is a suitable short-wavelength laser. Only a few refractive optical materials are appropriate for use in optical systems with this short wavelength. The most commonly used materials are synthetic fused quartz and fluorite. Both of these materials show a gradual decline in transmissivity when irradiated by light of intensities greater than certain threshold intensities. In order to prevent a decline in transmissivity, the optical systems of projection-exposure apparatus frequently enlarge the diameter of the light beam so as to reduce the intensity of optical pulses on the lenses.
This invention provides pulse-width extending optical systems, projection-exposure apparatus comprising such systems, and manufacturing methods using such projection-exposure apparatus. The pulse-width extending systems lower laser peak powers in the optical system without reducing average laser power. For convenience, optical pulse width is defined as the time during which an optical pulse has an intensity greater than one-half of the maximum value of the intensity. It is also convenient to define an optical pulse length as the distance traveled by an optical pulse in a time equal to its pulse width.
In a preferred embodiment of a pulse-width extending optical system according to the invention, a beamsplitter splits an incident laser pulse into two or more pulses. The pulses propagate along optical paths such that they are delayed with respect to each other. A beamsplitter then receives the delayed pulses and directs them along a common output optical path. In the example embodiments, one or more beamsplitters split and combine the pulses.
Because the split pulses are delayed, the peak power at the output is reduced because optical pulse energy from the original pulse arrives over a time period that is longer than the original pulse duration. The delay among the pulses is set by causing the split pulses to travel different optical paths. To effectively reduce the laser power, the optical path differences are preferably greater than the pulse length.
Pulse-width extending optical systems according to the invention also preferably comprise relay systems operable to ensure that the delayed pulses maintain appropriate beam cross-sections and do not become large because of the natural divergence of light beams. The relay systems provide an additional benefit. If a relay system inverts a beam image, then beam uniformity is improved because the relay system overlaps a beam and an inverted image.
According to another aspect of the present invention, projection-exposure apparatus are provided comprising pulse-width extending optical systems as summarized above. In such a projection-exposure apparatus, the pulse-width extended radiation is directed to a multi-source image mechanism to form multiple images of the pulse-width extended radiation. A condenser then illuminates the mask substantially uniformly using the multi-source images. This projection-exposure apparatus can be advantageously used for manufacturing semiconductor devices.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of the example embodiments which proceeds with reference to the accompanying drawings.